Soft errors involve changes to data (e.g., the electrons in a storage circuit), but do not necessarily indicate changes to a physical circuit. After a soft error has been observed, there is no implication that a circuit or component is unreliable. In other words, if data is rewritten after a soft error, a circuit can still function properly and the error may go unnoticed.
Background radiation from alpha particles, neutrons and cosmic rays can create soft errors in an IC. Momentary upsets (so-called single-event upsets, or SEUs) in data inside an IC may lead to a soft error. Some SEUs, called single-event transients (SETs), do not affect bit values. Other, more severe SEUs may affect the value of one or more bits. SEUs that affect the value of one bit are called SBUs. The rate at which SBUs occur affects the IC's SER. The SER of an IC is the rate at which soft errors occur due to, for example, background radiation.
SBUs may go unnoticed if the data is changed back to the correct value before it is stored. However, an SBU may cause a soft error if the upset data is stored or if the upset directly changes the data contained in a storage element. Modern technology is more susceptible to upsets due to reductions in feature sizes and operating voltages. With such reductions, the SER of standard logic elements in ICs continues to rise. Thus, reducing the SER associated with logic elements has become an important design consideration.
SER for ICs can be reduced at a system level or at an individual gate level. At the system level, products targeted for devices more likely to receive increased background radiation, such as satellites, have been designed to increase SER tolerance by using temporal or special displacement schemes, or by replicating logic and using voting schemes. Different designs have been used which trade off area, power performance and SER, although many of these designs are proprietary. System and architectural solutions to reduce SER require customer interaction which may not be practical (especially if the design is created in a traditional ASIC engagement) and can be expensive in terms of the IC die area required. A voting scheme, for example, may require three times the die size of a traditional design that is not enhanced to address soft errors. Due to additional requirements, conventional solutions to reduce SER are almost exclusively used for space, medical or military markets which can tolerate the high initial device cost to reduce the cost of a failing device.